1. Field of the Invention
The present invention relates to a magnetic recording transfer method for use with the combination of a mother magnetic recording medium and a copy magnetic recording medium of a metal-deposited thin film type each. More particularly, the invention relates to a magnetic recording transfer method suitable for transferring short wave length magnetic recordings between the so-called high band eight-millimeter format video tapes and for transferring digital video signal magnetic recordings between digital video signal tapes.
2. Description of the Related Art
In a magnetic recording bulk copying setup, large quantities of video and audio recordings are copied from their source onto magnetic recording media. Such production generally involves the use of what is known as the magnetic recording transfer method. Under this prior art method, a mother magnetic recording medium containing source recordings such as a mother tape is subjected to a biased magnetic field (i.e., transfer magnetic field). With the mother tape brought to the proximity of or contact with a copy magnetic recording medium such as a copy tape, the biased magnetic field copies the magnetic recordings from mother tape to copy tape.
One advantage of the above prior art magnetic recording transfer method is its capability to copy large quantities of recordings at high speed, hence in a short time.
Conventionally, the coercive force H.sub.C of the mother magnetic recording medium is held at about 2.5 times the coercive force H.sub.C of the copy magnetic recording medium. This measure is taken to minimize the demagnetization of the mother magnetic recording medium by the biased magnetic field and to make the transfer of recordings to the copy magnetic recording medium easier.
However, with such a high coercive force, the mother magnetic recording medium still has difficulty in eliminating the incidence of its demagnetization. One drawback of the conventionally used, metal-coated magnetic recording media is that they tend to deteriorate in the reproduction output of short wave length magnetic recordings that were placed thereon for high density recording. For example, assume that today's best material as the metal-coated magnetic tape (e.g., high coercive force metal-coated tape) is used as the mother tape in combination with a Ba-Fe tape as the copy tape. According to past tests and measurements, those eight-millimeter format video tapes of the above type which contain recordings in a short wave length range of 0.7 .mu.m (=.lambda.; recording wave length) generally yield a magnetic transfer output appreciably inferior to that of metal-deposited thin film type magnetic tapes.
Japanese Patent Laid-Open No. 57-138061 discloses a magnetic copying method for use with the combination of a master (i.e., mother) magnetic recording medium and a slave (i.e., copy) magnetic recording medium positioned opposite to each other for magnetic copying of recordings therebetween. Under this method, as shown in the schematic cross-sectional view of FIG. 21, the magnetic layer surface 4 of each magnetic recording medium is measured for residual magnetism in two directions A and B. Direction A is at 45.degree. with respect to the plane direction identified by the X axis, while direction B is at -45.degree. relative to the same plane direction. The residual magnetism of the mother magnetic recording medium 1 measured in direction B is at least 25% greater than that measured in direction A. The residual magnetism of the copy magnetic recording medium 2 measured in direction A is at least 25% greater than that in direction B. With the two media positioned as indicated, a biased magnetic field H.sub.B is applied in or approximately in direction A for magnetic copying.
The above magnetic copying method is used in connection with the magnetic layer of the medium made of a diagonally deposited magnetic metal. On the mother magnetic recording medium 1, the biased magnetic field H.sub.B is applied in the direction of its hard axis; on the copy magnetic recording medium 2, the biased magnetic field H.sub.B is applied in the direction of its easy axis. This scheme appears to make the mother magnetic recording medium 1 highly immune to demagnetization even if the coercive force thereof is relatively small. That in turn appears to make it possible to apply a sufficiently high level of biased magnetic field H.sub.B to the mother medium for enhanced copying efficiency.
However, the magnetic copying method disclosed in Japanese Patent Laid-Open No. 57-138061 has turned out to have some disadvantages regarding demagnetization of the mother magnetic recording medium during copying and regarding the efficiency in copying.
According to the invention disclosed in Japanese Patent Laid-Open No. 57-138061, the biased magnetic field H.sub.B is applied in direction A, i.e., at 45.degree. with respect to the in-plane direction of the magnetic recording medium. Direction A is supposed to coincide with the hard axis for the mother magnetic recording medium and with the easy axis for the copy magnetic recording medium. In fact, however, the easy axis for the copy medium is not formed in the 45.degree. direction.
Metal-deposited thin film magnetic recording media are generally formed as follows: The substrate of a magnetic recording medium, i.e., a long sheet of a nonmagnetic base is led onto the cylindrical surface of a guide drum. In this state, a magnetic metal material is deposited from a deposition source onto the base within an angular range of 90.degree. to 40.degree.. Magnetic growth grains resulting from deposition, or the so-called columns, are formed at about 45.degree. with respect to the medium surface. However, even when the columns are formed with the 45.degree. inclination, the fact that a strong diamagnetic force is exerted perpendicularly to the metal-deposited thin film medium causes the magnetizing vector to incline less in the crystallizing direction (i.e., column growing direction) than in the in-plane direction. Thus the actual easy axis is not aligned precisely with the 45.degree. direction. When the biased magnetic field H.sub.B is applied in the -45.degree. direction as described, the direction of magnetic field application is not aligned with the hard axis of the mother magnetic recording medium or with the easy axis of the copy magnetic recording medium. As a result, it is impossible to obtain a satisfactory reduction of the demagnetizing effect on the mother magnetic recording medium or a high enough increase in magnetic copying output. In practice, where magnetic recordings are transferred from one medium to another within a short wave length range of about 0.7 .mu.m as described above, there is a possibility of promoting demagnetization of the mother magnetic recording medium or of reducing the efficiency in magnetic recording transfer.
There are two kinds of digital VTRs (video tape recorders) for today's broadcast use: the D1 format component type digital VTR and the D2 format composite type digital VTR. These VTRs operate on the method of recording digitized color video signals onto a recording medium such as magnetic tapes.
The D1 format digital VTR converts a luminance signal and color difference signals (first and second) from analog to digital format at sampling frequencies of 13.5 MHz and 6.75 MHz, respectively, and records the digital format signals onto a magnetic tape after subjecting them to necessary signal processing. Because the sampling frequencies for the component luminance signal, first color difference signal and second color difference signal are 4:2:2 in ratio, this VTR is also called the 4:2:2 system.
The D2 format digital VTR converts composite color video signals from analog to digital format at a sampling frequency four times the color subcarrier signal frequency f.sub.sc, and records the digital format signals onto a magnetic tape after subjecting them to necessary signal processing.
Since these VTRs are primarily designed for use by broadcasting stations, the highest priority of the VTR facility is the quality of broadcast pictures. When digital color video signals are recorded, each sample thereof is illustratively converted to eight bits with no substantial data compression taking place.
As an example, the amount of data handled by the D1 format digital VTR is discussed below.
When color video signals are converted from analog to digital format at the above-mentioned sampling frequencies, eight bits processed together per sample, the amount of the color video signal information involved is about 216 Mbps (megabits per second). With the horizontal and vertical blanking period data excluded, the effective pixel count per horizontal period is 720 for the luminance signal and 360 for the color difference signals. With the NTSC system (525/60) in use, the effective scanning line count per field is 250. Thus the video signal data amount Dv per second is ##EQU1##
With the PAL system (625/50) in use, the effective scanning line count per field is 300 and the field count per second is 50. It can be seen from this that the PAL system handles the same amount of data as the NTSC system. When redundant data components are added for error correction and formatting purposes, the overall bit rate of video data amounts to about 205.8 Mbps.
Audio data Da totals up to about 12.8 Mbps. Additional data Do such as gaps, preambles and postambles for editing purposes amounts to about 6.6 Mbps. Thus the NTSC system has the total amount of recording data Dt totaled up to ##EQU2##
To record the above data amount, the D1 format digital VTR utilizes 10 tracks per field when working with the NTSC system and 12 tracks per field with the PAL system (segment scheme).
The recording tape in present use is 19 mm wide. The tape comes in two thicknesses: 13 .mu.m and 16 .mu.m. There are three different cassettes to accommodate the tapes: large type, medium type and small type. When the data is recorded in the above-described format on the tape, the data recording density thereon is approximately 20.4 .mu.m.sup.2 per bit.
With the above-described parameters taken into account, the reproduction times of the respective cassettes run by the D1 format digital VTR are: 13 and 11 minutes for tape thicknesses of 13 and 16 .mu.m respectively with the small size cassette; 42 and 34 minutes for tape thicknesses of 13 and 16 .mu.m respectively with the medium size cassette; and 94 and 76 minutes for tape thicknesses of 13 and 16 .mu.m respectively with the large size cassette.
As described, the D1 format digital VTR offers sufficient capabilities for broadcast use with the highest emphasis on picture quality. However, this VTR has its share of disadvantages, one of which is a short reproduction time. The large size cassette containing a 19-mm wide tape reel provides only about 1.5 hours of reproduction when run by the D1 format VTR. This makes it difficult to adapt this VTR to household use.
On the other hand, if signals of wave lengths as short as 0.5 .mu.m are recorded on a track 5 .mu.m wide, a recording density of 1.25 .mu.m.sup.2 per bit is achieved. Thus if the recording operation is performed in conjunction with a suitable data compression method that compresses recording data with minimum reproduction distortion, long-hour recording and reproduction are available even with tapes whose width is eight millimeters or less.
However, because the recording wave length is shortened to 0.5 .mu.m for higher recording density, transferring magnetic recordings from the mother tape to the copy tape results in a very low level of transfer output on the copy tape side. For this reason, it is difficult to reproduce in large quantities digital VTR copy tapes that are required to provide high output levels.